Polishing System with Front Side Pressure Control

ABSTRACT

A polishing apparatus includes a platen having a first surface to support a polishing pad and a second surface, a carrier head to hold a substrate against the polishing pad, a plurality of through-holes defined in the platen, and a pad pressure control assembly adjacent on a side of the platen opposite the carrier head.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is claims priority to U.S. Application Ser. No.61/801,163, filed on Mar. 15, 2013, which is incorporated by referencein its entirety.

TECHNICAL FIELD

This disclosure relates to the architecture of a chemical mechanicalpolishing (CMP) system.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. One fabrication step involves depositing afiller layer over a non-planar surface and planarizing the filler layer.For certain applications, the filler layer is planarized until the topsurface of a patterned layer is exposed. A conductive filler layer, forexample, can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. After planarization, theportions of the metallic layer remaining between the raised pattern ofthe insulative layer form vias, plugs, and lines that provide conductivepaths between thin film circuits on the substrate. For otherapplications, such as oxide polishing, the filler layer is planarizeduntil a predetermined thickness is left over the non planar surface. Inaddition, planarization of the substrate surface is usually required forphotolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is typically placed against a rotating polishing pad.The carrier head provides a controllable load on the substrate to pushit against the polishing pad. An abrasive polishing slurry is typicallysupplied to the surface of the polishing pad.

Some carrier heads include multiple independently pressurizable chambersso that pressure on different regions of the back surface of thesubstrate, i.e., the surface that is not being polished, can becontrolled.

SUMMARY

In one aspect, a polishing apparatus includes a platen having a firstsurface to support a polishing pad and a second surface, a carrier headto hold a substrate against the polishing pad, a plurality ofthrough-holes defined in the platen, and a pad pressure control assemblyadjacent on a side of the platen opposite the carrier head.

Implementations may include one or more of the following features.

The pad pressure control assembly may include a plurality ofindependently controllable pad pressure control elements. The apparatusmay include a plurality of fluid channels, each channel configured todirect a fluid from an output end of the channel to one or more of theplurality of through-holes. A pressure control assembly may beconfigured to direct the fluid through the fluid channels at sufficientpressure to press on the polishing pad supported on the first surfaceand deform a portion of the polishing pad that is positioned above thethrough-hole.

A bearing ring may surround one or more of the plurality of pad pressureelements. The bearing ring may be a contact bearing ring to contact abottom surface of the platen. The bearing ring may be a fluid bearingring to maintain a gap from a bottom surface of the platen. The bearingring may be configured to adjust the distance between the pad pressurecontrol elements and the second surface of the platen.

A linear rail may support the plurality of pad pressure elements. Theplurality of pad pressure elements may be movable on the linear rail ina first direction in a plane parallel to the first surface of theplaten, the plane being the first distance away from the platen. Asecond linear rail may support the plurality of pad pressure elements,and the plurality of pad pressure elements may be movable along thesecond rail in a plane parallel to the first surface of the platen in asecond direction perpendicular to the first direction.

The plurality of pad pressure control elements may be magnets, eachmagnet being positioned in one of the plurality of through holes. Aplurality of electromagnets and a controller may be configured tocontrol a current applied to the electromagnets to exert a controllableamount of force on a corresponding magnets positioned in thethrough-holes. A magnet may be attached to the carrier head andconfigured to exert a force on the magnets positioned in thethrough-holes.

The pressure control assembly may include a plurality of independentlycontrollable annular control zones. The plurality of independentlycontrollable annular control zones may include a plurality of concentricelements each having a different radius, the plurality of concentricelements being positioned in a plane parallel to the platen, the planebeing a first distance away from the second surface of the platen, theplurality of elements being closer to the second surface than the firstsurface. The plurality of independently controllable annular controlzones may be segmented. The plurality of independently controllableannular control zones may be continuous.

In another aspect, a method of polishing includes supporting a polishingpad on a platen, holding a substrate in a carrier head with a surface ofthe substrate against a top surface of the polishing pad, applying apressure on a bottom surface of the polishing pad from a plurality ofthrough-holes defined in the platen, and controlling the pressure with apad pressure control assembly adjacent the platen on a side of theplaten opposite the carrier head.

In another aspect, a polishing apparatus includes a carrier head to holda substrate against a polishing pad having a polishing surface incontact with the substrate, and a plurality of pad pressure controlelements. The carrier head includes a retaining ring, and the controlelements are arranged in a plane below a polishing surface of thepolishing pad and configured to be activated by proximity of theretaining ring.

Implementations may include one or more of the following features.

The retaining ring may include a magnetic material. The magneticmaterial in the retaining ring may be configured to activate selectedcontrol elements when the magnetic material is vertically above theselected control elements.

The control elements may be magnets. The magnets may be positioned in alayer of flexible material. The magnets may be positioned in a pluralityof through-holes in the platen.

The control elements may be actuators supported by the platen. Theactuators may be connected to respective power sources and configured tobe driven by a controller to drive the actuators in a verticaldirection.

The control element may be arranged in a regular pattern. The patternmay cover spans an upper surface of the platen. The control elements mayprovide an active matrix layer.

In another aspect, a method of polishing includes supporting a polishingpad on a platen, holding a substrate in a carrier head with a surface ofthe substrate against a top surface of the polishing pad, retaining thesubstrate below the carrier head with a retaining ring, and applying apressure on a bottom surface of the polishing pad with a plurality ofpad pressure control elements, wherein the control elements areconfigured to be activated by proximity of the retaining ring.

Implementations can include one or more of the following potentialadvantages. Pressure on the substrate can be controlled from the outersurface of the substrate. The control pressure can be transferredefficiently to change the interface pressure of the polishing pad in thearea of retaining ring and substrate edge. A post removal profile of thesubstrate can be more accurately controlled.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other aspects, featuresand advantages will be apparent from the description and drawings, andfrom the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional side view of selected elements of a CMPapparatus.

FIG. 2A schematically illustrates pressure distribution in a stiffsubstrate.

FIG. 2B schematically shows pressure distribution in a soft substrate.

FIG. 3A schematically shows pressure distribution on a substrate outersurface in a carrier head.

FIG. 3B-3C show embodiments that include an active matrix layer.

FIG. 3D shows an embodiment that include actuators.

FIGS. 4A-4I show embodiments using a pneumatic pressure control system.

FIGS. 5A-5B show embodiments using a magnetic pressure control system.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows a cross-sectional side view of selected elements of the CMPapparatus 10. The polishing apparatus 10 includes a platen 16 to supporta polishing pad 18, and a carrier head 12 to hold a substrate 14 againsta polishing surface 181 of the polishing pad 18.

The carrier head 12 can include a retaining ring 152 to retain thesubstrate 14 below a support structure 154, such as a flexible membrane.The substrate 14 has an inner surface 141 abutting the supportstructure. The carrier head 12 can control the polishing parameters, forexample pressure, used to polish the substrate 10. For example, thecarrier head 12 can includes a plurality of independently controllablepressurizable concentric chambers defined by the membrane, e.g., threeconcentric chambers 146 a-146 c, which can apply independentlycontrollable pressurizes to associated zones on the flexible membrane154 and thus on the substrate 14. Although only three chambers areillustrated in FIG. 1 for ease of illustration, there could be one ortwo chambers, or four or more chambers, e.g., five chambers. Inaddition, although only one carrier head is illustrated, there can bemore than one carrier head.

In operation, the carrier head 12 holds a substrate 14 to be polishedagainst the polishing surface 181 of the polishing pad 18 so that anouter surface 142 of the substrate 14 is in contact with the polishingsurface 181. The inner surface 141 is in contact with the membrane 154.

The platen 16 has a top surface 161 and a bottom surface 162. The topsurface 161 supports the polishing pad 18. A bottom surface 182 of thepolishing pad 18 is in contact with the top surface 161 of the platen.The polishing pad 18 can be a two-layer polishing pad with a polishinglayer 112 and a backing layer 114. The polishing pad 18 can be securedby an adhesive layer 28 to the platen. The adhesive layer 28 can be adouble-sided adhesive tape, e.g., a thin layer of polyethyleneterephthalate (PET), e.g., Mylar™, with adhesive, e.g.,pressure-sensitive adhesive, on both sides.

The pressure with which a portion of the substrate 14 is pressed againstthe polishing surface 181 of the polishing pad 18 on the rotating platen16 determines the polishing rate experienced by that portion of thesubstrate. This pressure is also an interface pressure between thesurface 142 and the polishing surface 181. The profile of the substrateafter polishing can also be referred to as the post removal profile. Byapplying different pressures to different portions of the substrate, thea substrate can be polished to achieve a desired post removal profile.For example when polishing an incoming substrate having uneven surfacefeatures (e.g., different surface heights across the substrate), theinterface pressure profile across the substrate can be controlled toremove different amount of material across the surface of the substrateso that the post removal profile of the substrate results in a flatsurface. Due to the rotation of the carrier head about the axis 32, theprofile of the polished substrates will tend to be substantiallyrotationally symmetric.

When pressure is applied on the substrate 14 from the inner surface 141,the pressure is transmitted through a relatively stiff substrate 14(e.g., a silicon wafer) to press the outer surface 142 of the substrateagainst the polishing surface 181. As shown in FIGS. 2A and 2B, a stiffsubstrate 14 will “redistribute” a localized pressure 90 across aportion of the lower outer surface 142 below and around the applicationpoint of the localized pressure 90. In contrast, layer 91 of a softmaterial performs less “redistribution” of the applied pressure, andtransmits the pressure to a smaller localized region on an oppositesurface 92.

Adjacent concentric chamber zones 146 a-146 c in the polishing head arecontiguous without significant gaps or overlaps between the zones.However, as a result of the redistribution of pressure by the substrate14, when different pressures are applied in adjacent regions from theinner surface of the substrate, there is a gradual transition betweenthe regions in the applied pressure on the outer surface.

A nominal transition width 192 is obtained when different pressures areapplied to adjacent regions of the inner surface 141 of the substrate 14(see FIG. 3A). A nominal interface pressure profile 194 may have a widertransition width than desired. It may be desirable to obtain a narrowertransition width at the interface between two independently controllableregions. For example, it may be desirable to have narrower transitionbetween an edge region 143 and a central region.

In some implementations, the edge region 143 may include the outermost20 mm from the outer perimeter of the substrate. For a carrier head 12having two concentric pressure chambers, a large central chamber 201 anda smaller rim chamber 202, the interface pressure profile experienced bythe substrate will include a uniform (i.e. flat) portion in the centerof the substrate and a transition region around the substrate edge(e.g., 20 mm from the outer perimeter of the substrate). Non-uniforminterface pressure profile features tend to occur in a sub-region of thesubstrate edge (e.g., 10 mm from the edge), and due to the need for thepressure to be transmitted through the rigid substrate, fine interfacepressure profile control of the zone transition at the substrate edge isdifficult. The total width of the zone transition on both sides of thesubstrate when pressure is transmitted from the inner surface 141 to theouter surface 142 is 20 to 30 mm.

Instead or in addition to applying pressure on the substrate 14 from theinner surface 141, pressure can be applied directly to the outer surface142 of the substrate 14. In particular, pressure is applied through thepolishing pad 18.

A potential advantage of this configuration is that it can achieve anarrower transition width. Narrower transition widths can be obtainedwhen the interface pressure is applied directly at specific locations onthe outer surface 141 without having to have the pressure be transmittedthrough the rigid substrate 14.

The polishing system 10 includes a pressure control assembly that cancontrol pressure applied upwards to the underside of the polishing pad18. In some implementations, the pressure control assembly includes aplurality of control elements that are operable to apply a pressure tothe underside of the polishing pad 18. The pressure is appliedperpendicular to the top surface 161 of the platen or the outer surface142 of the substrate 14 when held by the carrier head 12. The controlelements can be uniformly spaced below the polishing pad. The controlelements can cover at least any lateral area that the substrate canoverlie during the polishing operation, e.g., substantially all of thepolishing pad.

In some implementations, the control elements are movable in response toan applied magnetic force, e.g., from a magnet. For example, the controlelements can be movable vertically, i.e., perpendicular to the topsurface 161 of the platen or the outer surface 142 of the substrate 14when held by the carrier head 12, in order to press against theunderside of the polishing pad 18. In some implementations the controlelements are movable vertically but are laterally restrained. In someimplementations, the vertical motion of the control elements isindependently controllable.

FIGS. 3B-3C show an active matrix layer (AML) 390 (FIG. 3B is apartially exploded view). The AML 390 can provide a narrower transitionwidth for finer post removal profile control. The AML 390 can besupplied in an independent sub-pad 399 or it can be incorporated intothe polishing pad 18. The AML 390 includes control elements (e.g.,391,392, 393) that are embedded and laterally distributed within the AML390 to form a layer of flexible motion elements. The control elementscan be uniformly spaced through the AML 390. The AML 390 could befabricated within an upper portion of the platen 16 or it could beadhered to the upper surface 181 of the platen.

A retaining ring 388 in the carrier head 12 can have a magnet 387, e.g.,a permanent magnet or electromagnet 387, to interact with controlelements 391-393 embedded in the AML 390. Each control element 391-393can be a permanent magnet or a ferromagnetic material that can responseto the magnetic field produced by magnet 387 carried by the modifiedretaining ring 388. The control element 391-393 can range in size fromthe micrometer scale to about 10 mm. The control element 391-393 canalso be coils. The Lorentz force produced by the magnet 387 in themodified retaining ring manipulates an electric current in the coil. Insome embodiments, it is possible to incorporate control elements in aregion 386 of the modified carrier head to control the post removalprofile in the central portions of the substrate.

During operation, the platen 16 and carrier head 12 rotate about theirrespective axis. As shown in FIG. 3C, the control element 391 below themodified retaining ring 388 is magnetically excited and translatesvertically. The control element 391 will move towards or away from themagnet 387 in the retaining ring 388 depending on the relativeorientation of the poles of the magnet 387 and the control element 391,i.e., toward if opposite poles are adjacent and away if similar polesare adjacent.

The movement of the control element 391 within the AML 390 changes theinterface pressure of the polishing pad in the area of directlyunderneath the retaining ring 388 and also in the surrounding regions384 and 385. The surrounding region 385 overlaps with a portion 384 ofthe polishing surface 181 that is in contact with an edge portion 143 ofthe substrate, and the interface pressure at that edge portion caneither increase or decrease, allowing finer control of the post removalprofile of the substrate.

The interface pressure can be increased when the control elementtranslates upwards, compressing the polishing pad and pushing thepolishing surface 181 closer to the substrate. Conversely, when thecontrol element translates downwards, the polishing pad is stretched andthe polishing surface 181 is pulled away from the substrate 14, loweringthe interface pressure at the edge region of the substrate. The controlelement can translate downwards when a polarity of the electromagnet 387in the modified retaining ring is reversed. A lower interface pressureat a particular location decreases the removal rate of the substrate inthat region. The interface pressure of a lateral extent 383 of theregion of polishing pad 18 can be controlled by the magnitude of thevertical displacement of the control element. A larger displacementwould result in a larger region over which a transition zone of thesubstrate can be controlled. The activated control elements aredeactivated when the retaining ring moves laterally away from thecontrol element.

In some embodiments, the control elements can be actuators, e.g.,piezeoelectric actuators. When activated, the actuators extend orcontract along a vertical axis, i.e., perpendicular to the top surface161 of the platen or the outer surface 142 of the substrate 14 when heldby the carrier head 12. The actuators can be uniformly spaced across atwo-dimensional area below the polishing pad. Alternatively, theactuators can be arranged along a radial dimension 164 of the platen.

As shown in FIG. 3D, instead of an AML 390, an upper portion of theplaten 16 can include a number actuators 379-382. Although FIG. 3D showsonly four actuators, there could be more actuators and the actuatorscould be uniformly spaced across the entire polishing pad.

The modified retaining ring 388 activates the actuators that positionedunder retaining ring or another designated area. When actuatorsactivate, another power source (375-378) will be used to drive theactuators (up or down) to create higher/lower interface pressure betweenthe polishing surface 181 and the outer face 142 of the substrate 14.

The linear response of the control elements 391-393 and the actuators379-382 ensures that the interface pressure can be effectively controlbecause an area of interest where higher/lower interface pressure isrequired is not stationary but is dynamically changing. Linear responsehelps to ensure that no parasitic nonlinear responses persist in thecontrol element or the actuators remains when the modified retainerrings exits a particular region, and also ensures that the elements andactuators can be activated quickly when the modified retainer ringenters a particular region.

In some embodiments, the control elements can be fluid pressure sources.When activated, each pressure sources pressurize or depressurize fluidin a respective volume below the polishing pad.

Thus, instead of or in addition to the AML 390, the interface pressureat the outer face 142 of the substrate 14 can be controlled by apneumatic pressure control assembly 300 shown in FIG. 4A-4G. A pattern200 of through-holes 210 is defined within the platen 16 (shown in FIG.4A). The platen 16 has a radial dimension 164. FIG. 4A shows a top viewof selected components of the CMP apparatus 10. The polishing pad 18 isnot illustrated in FIG. 4A in order to show features of the platen 16.

FIG. 4B shows a top view of the pneumatic pressure control assembly 300includes a body 310, e.g., a disc, having a top surface 311 and a bottomsurface 312. FIG. 4C shows a cross sectional view of the pneumaticpressure control assembly 300. Channels (e.g., 321 and 322) which areperpendicular to and extend through the top and bottom surface 311 and312 are defined in the body. A fluid outlet 331 of a fluid deliverysystem 330 having a fluid source 332 is directed at the channel 321 fromthe bottom surface 312. A controller 333 is able to control the amountof fluid entering each individual channel defined in the body 310. Thefluid in the channel then travels up a corresponding through-hole 210 inthe platen 16. The corresponding through hole is one that issubstantially vertically aligned with a specific channel in the body.The flow of fluid up the through hole from the lower surface 162 of theplaten to the upper surface 162 exerts a force on the surface 182 of thepolishing pad 18. This force deflects the surface 182 and in theprocess, exerts an interface pressure between the polishing surface 181and the outer surface 142 of the substrate 14.

In some implementations, during polishing, the carrier head 12translates along a lateral dimension 164, e.g., along the radius of theplaten 120. For example, the carrier head 12 can move along a track 128or be carried by a carousel, while the platen 16 rotates. The lateraltranslation of carrier head 12 allows the substrate 14 to more evenlyand fully utilize available surface for polishing on the pad 18.

The body 310 in the pneumatic pressure control assembly 300 can bemounted on a rail 35 that follows the lateral motion of the carrier head12. For example, the rail 35 also extends along the radial dimension 164of the platen 16. A controller 95, e.g., a computer, can synchronize thelateral translations of both the carrier head 12 and the body 310 sothat they track each other. This ensures that the radially controllablezones of the body 310 remain aligned with their respective regions ofthe substrate 14 as the substrate moves laterally.

The distance between adjacent channels in the body 310 may range from afew millimeters (e.g., 1 mm, 2 mm) to hundreds of millimeters (e.g., 100mm). Even though the fluid deflects the lower surface 182 of thepolishing pad 18 at discrete locations (i.e. at locations vertical abovethe corresponding through hole 210 within which the fluid flows), due tothe radial translation of the carrier head 12, the effect of theincrease in interface pressure can be averaged over a determined widthof a particular annular zone on the outer surface of substrate 14.

The pneumatic pressure control assembly 300 can include a bearing ring350 that surrounds the body 310. In some implementations, the bearingring 350 can be a fluid bearing ring. For example, the body and bearingring 350 can be biased upwardly, but fluid injected between the body 310and the platen 16 to maintain a vertical gap between of the body 310 andthe bottom surface of the platen. Alternatively, rather than a fluidbearing ring, a contact bearing ring 350 can surround the body 310. Thecontact bearing ring 350 can be formed out of a low-wear material, e.g.,a plastic.

Alternatively, the position of the body 310 can be fixed in the zdirection. The fluid bearing ring 350 can be further coupled to anactuator 370 that translates along the z-direction such that a distance365 along the z-direction between the top surface 301 and the bottomsurface 162 of the platen 16 can be controlled (see FIG. 4E).

The top surface 301 of the body 310 does not contact surface 162 of theplaten in order to reduce wearing out of the surfaces 301 and 162 fromfriction, and to allow the platen 16 to rotate above the body 310 forthe substrate 14. The z-axis position of the body 310 can also bemanipulated to control the amount of deflection of the polishing pad 18.For example, for an equal amount of fluid entering the channel 311 fromthe bottom surface 311 of the body 310, a larger distance 365 reducesthe deflection imparted because some fluid may escape into the gapdefined by the distance 365 resulting in a smaller amount of fluid thatactually enters the through hole 210.

The entire assembly of the body 310 and the fluid bearing ring 350 canbe supported by a bracket 360 that is mounted on a stationary point 395on the rotating shaft 124 of the platen 16.

A metrology unit 398 is used to measure optical characteristics of thesubstrate 14. The metrology unit 398 can be positioned in and rotatewith the platen 16. The metrology unit 398 can be located in a region ofthe platen 16 without through-holes 210.

In general, the platen 16 may contain more through-holes 210 than thenumber of corresponding channels in body 310. In some embodiments, somethrough-holes 210 will be “inactive”—no fluid is flowing through thosethrough holes to deflect the surface 182 of the polishing pad 18.

Each channel in the body 310 can have its own associated fluid outletfrom the fluid delivery system 320. Alternatively, all the channels atthe same radial distance from a center of the body 310 can share asingle fluid outlet 312. For example, FIG. 4F shows a stretched out viewalong an arc 371 of the body 310. A channel 366 which receives fluid 399from the fluid delivery system 330 extends through both the top andbottom surfaces of the body 310. A horizontal channel 372 connectsvarious channels 367 formed at the same radial distance. Channels 367have ends 368 that open at the top surface 311 of the body 310 but donot have openings at the bottom surface 312 of the body 310. Thehorizontal channel 372 directs fluid received from fluid outlet 312 tothe open ends of channels 367 and out into a corresponding through hole210 in the platen 16.

The through holes 210 defined in the platen 16 may have across-sectional dimension of 1 to 20 mm. The cross-sections may havecircular, oval, polygonal or any other irregular shapes.

Instead of an integral body 310, the pneumatic pressure control system300 can have discrete concentric rings 316, 317 and 318 each having adifferent radius, as shown in FIG. 4G. Each ring has a number ofchannels defined therein and the channels may extend through both thetop and bottom surfaces of each respective ring, or they may beconfigured in a fashion similar to the arrangement depicted in FIG. 4Fwhere each ring has a single inlet on the bottom surface of the ring andmultiple outlets on the top surface of the ring closer to the surface162 of the platen. Instead of a complete ring 315, the pneumaticpressure control system 300 can include partial segments 325-329 of aring, the partial segments being arcs of circles having different radii.As described earlier, due to the rotation of platen the 16, the forceimparted by fluid traveling through channels in segments 325-329 andalong their corresponding through holes 210 in the platen 16 will resultin a substantially rotationally symmetric post removal profile of thepolished substrate 14.

In some implementations, each control element, e.g., each channelthrough the platen, can have its own bearing. For example, as shown inFIGS. 4H and 4I, each channel 321, 322, etc., can be surrounded by abearing 440. Alternately, each a group of channels with a commonlycontrolled pressure from the same fluid outlet 331 can have its ownbearing. The bearing can project upwardly toward the platen 16 from thetop surface 311 of the body 310. Each bearing 440 can laterally surrounda respective channel. The bearing 440 can be a contact bearing, in whichcase the bearing contacts the bottom surface of the platen 16, or afluid bearing, in which case a small gap remains between the bearing andthe platen.

FIGS. 5A and 5B show a magnetic control system 500. A series ofconcentric annular electromagnets 401-403 having different radii isarranged below the platen 16. Each electromagnet is connected toelectronic circuitry 406 that allows each electromagnet to be controlledindividually to create magnetic fields of varying different magnitude asa function of time. The through holes in 210 in the platen 16 eachcontain a magnet (e.g., 411, 412). A polar axis is the vertical linepassing through the magnet that connects the north and south poles ofthe magnet. The magnet 411 is arranged with its polar axis 422 parallelto the through hole 210. The electromagnets are also configured so thatwhen an electrical current passes through the electromagnet, its polaraxis 423 is parallel to polar axis 422. Instead of an electromagnet, apermanent magnet may also be used.

The magnet 411 is arranged so that when the electromagnet 401 isactivated, the poles of the electromagnet 401 and the magnet 411 thatare facing each other (i.e., the closest poles between the electromagnet401 and the magnet 411) have the same polarity. As a result, theelectromagnet 401 and magnet 411 repel each other. Due to the annularelectromagnet 401 being fixed in place by the support 460, when asufficiently large magnetic field is generated by the electromagnet 401,the magnet 411 is propelled upwards through the opening of the throughhole 210 at the surface 160 of the platen 16. The magnet 411 then pusheson the bottom surface 182 of the polishing pad, deflecting it andincreasing the interface pressure between the substrate and thepolishing pad at a corresponding region vertically above the magnet 411.In FIG. 5B, electromagnet 402 is not activated while electromagnet 401is activated. The electromagnet 401 propels the corresponding magnet 411above the surface 161.

Magnetic force decreases quadratically with distance. If the magneticfield created by the electromagnet is strong enough, then the series ofconcentric annular electromagnets can be attached to a rail 435 whichserves a similar function as the rail 35 in the pneumatic pressurecontrol system 300. Fixing the electromagnets on a rail this wayincreases a distance 564 between the surface 162 of the platen and thetop surface of the electromagnet. When the magnetic field generated bythe electromagnets is not strong enough, the concentric series ofelectromagnets can be mounted by a bracket and fixed to the stationarypoint in the driving shaft 124 of the platen 16, in a similar fashion asthat described above for the pneumatic pressure control system 300. Whenthe series of electromagnets are mounted to the bracket, a fluid bearingring 450 which serves the same function (i.e., z-axis control) surroundsthe concentric electromagnets.

In some embodiments, the through holes 210 are perpendicular to andextend through the first and second surfaces 161 and 162 of the platen16, as is the case for the platen 16 used with the pneumatic pressurecontrol system 300. In such embodiments, to prevent the magnet 411contained within each through holes 210 from falling out under theinfluence of gravity, a retaining sheet 163, e.g., of plastic, can beattached to the lower surface 162 so that the through holes 210 is notopen to the surface 162 but is terminated by the retaining sheet 163. Inthis case, the magnet 411 does not fall out from the through hole 210even when the electromagnet 410 is inactive (i.e., no current passesthrough the electromagnet 410).

The pattern 200 formed by the through holes 210 can be a regular (e.g.,Cartesian or polar) grid or it may be a non-regular pattern. The throughholes in the magnetic pressure control system are typically cylindricalin shape to accommodate the magnet 411.

The polishing pad can include a softer backing layer 110 that definesthe lower surface 182 of the polishing pad 18 (i.e., a relativelycompressible layer, such as a Suba-IV layer (from Rodel, Phoenix Ariz.).As described in FIGS. 2C and 2D, when the magnet 411 protrudes from thesurface 161 the pressure is effectively transferred to a region on thepolishing surface 181 directly in the vicinity above the magnet,allowing an increase in interface pressure on the outer surface 142 ofthe substrate 14 which is being polished.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other implementations are within the scope of the followingclaims.

1. A polishing apparatus, comprising: a carrier head to hold a substrateagainst a polishing surface of a polishing pad, wherein the carrier headincludes a retaining ring; and a plurality of pad pressure controlelements, the control elements arranged in a plane below the polishingsurface of the polishing pad and configured to be activated by proximityof the retaining ring.
 2. The polishing apparatus of claim 1, whereinthe retaining ring comprises a first magnet.
 3. The polishing apparatusof claim 2, wherein the first magnet in the retaining ring is configuredto activate selected control elements when the first magnet isvertically above the selected control elements.
 4. The polishingapparatus of claim 1, wherein the control elements are magnets.
 5. Thepolishing apparatus of claim 4, wherein the magnets are positioned in alayer of flexible material.
 6. The polishing apparatus of claim 4,wherein the magnets are positioned in a plurality of through-holes inthe platen.
 7. The polishing apparatus of claim 1, wherein the controlelements are actuators supported by the platen.
 8. The polishingapparatus of claim 7, wherein the actuators are connected to respectivepower sources and configured to be driven by a controller to drive theactuators in a vertical direction.
 9. The polishing apparatus of claim1, wherein the control elements provide an active matrix layer.
 10. Amethod of polishing, comprising: supporting a polishing pad on a platen;holding a substrate in a carrier head with a surface of the substrateagainst a top surface of the polishing pad; retaining the substratebelow the carrier head with a retaining ring; and applying a pressure ona bottom surface of the polishing pad with a plurality of pad pressurecontrol elements, wherein the control elements are configured to beactivated by proximity of the retaining ring.
 11. The method of claim10, wherein the retaining ring comprises a magnetic material.
 12. Themethod of claim 11, wherein the magnetic material activates selectedcontrol elements when the magnetic material is vertically above theselected control elements.
 13. The polishing apparatus of claim 3,wherein the control elements are a plurality of second magnetsconfigured to be attracted or repelled by the first magnet.
 14. Thepolishing apparatus of claim 3, wherein the first magnet comprises apermanent magnet.
 15. The polishing apparatus of claim 3, wherein thefirst magnet comprises an electromagnet.